Chemical nickel plating of magnesium and its alloys



States Patent ice L 9 2,983,634 l I CHEMICAL NICKEL PLATING F MAGNESIUM L ANDITSALLOYS Pranas mummies, Gary, 1nd,, assignor to General American Transportation Corporation, Chicago, 111., a corporation of New York Nb Drawi g. riled May 13,- 195 8,Ser. No. 734,823 as Claims. or, 148-6116 A still further object of the invention is to provide an improved process of producing, as an article of manufacture, a work-piece having a metal outer surface formed essentially of magnesium alloy and carrying a smooth and continuous coating intimately bonded thereto and essentially comprising a nickel-phosphorus alloy.

Further features of the invention pertain to the particular arrangement of the steps of the process, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood as the following specification proceeds.

In accordance with the present invention, there is provided a work-piece of article of manufacture having an outer surface formed essentially of magnesium, or an alloy thereof, and ordinarily the work-piece is first machined,

. or otherwise finished, and thereafter subjected to the areas therebelow of no adhesion and complete separation from the adjacent metal surfaces of the work-pieces.

Perhaps the principal reason for this lack of adhesion between this metal surface and the coating is that the metal surface is highly susceptible to oxidation and filmsover prior to or upon contact with the aqueous chemical nickel plating bath; whereby there is a film at the interface between the magnesium alloy metal surface and the coating ofnickel-phosphorus alloy that is inherently produced by a plating bath of this type; which film at the interface prevents proper initiation of the plating and intimate bonding between the magnesium alloy metal surface and the coating, with the result that the coating is characterized by blisters and other defects evidencing the lack of continuity and uniformity of adhesion thereof with respect to the metal surface. of the work-piece.

process so as to provide on the metal surface thereof a continuous, uniform and smooth coating intimately bonded thereto and formed of the nickel-phosphorus alloy that is inherently produced by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type. Specifically, after the work-piece is machined, or otherwise finished, it is first subjected to a pretreatment, then it is subjected to chemical deposition from a plating bath of the type noted, and ultimately it is subjected to a heat-treatment process in order to render the coating produced in the chemical nickel plating step intimately bonded thereto and of finished character.

Accordingly, it is a general object of the invention to provide a process of chemical nickel plating of a workpiece having a metal surface formed essentially of magnesium, wherein the coating produced is intimately bonded to the metal surface and exhibits great adhesion thereto.

Another object of the invention is'to provide a process of preparing the magnesium alloy metal surface of a work-piece for plating with a chemical nickel plating bath, so that the subsequently produced nickel-phosphorus alloy coating thereon is intimately bonded thereto and devoid of blisters and other defects characteristic of lack of adhesion.

Another object of the invention is to provide a process of chemical nickel plating upon a previously prepared magnesium alloy surface of a work-piece thatis productiveof a continuous smooth coating of nickel-phosphorus alloy thatis devoid of blisters and other defects characteristic of lack of adhesion.

A further object of the invention is to provide a process of the character noted that employs a chemical nickel plating bath of the nickel cation-hypophosphite anion type of improved and simple composition.

Another object of the invention is to provide a process of the character noted that utilizes an improved lowtemperature heat-treating step following the chemical nickelplating of the coating upon the metal surface of the work-piece, whereby the coating is intimately bonded to the underlying metal surface of the work-piece.

A further object of the invention is to provide a chemical nickel plating bath of the nickel cation-hypophosphite anion type and of improved composition.

Specifically, the work-piece is subjected to a Pretreatment No. 1 or a Pretreatment No. 2, depending upon the composition thereof. Pretreatment No. 1 is specifically recommended for magnesium alloys that are low in both aluminum and zinc (for example: AZ-31), and Pretreatment No. 2 is especially recommended for magnesium alloys that are high in either aluminum or zinc (for ex ample: ZK6OA, AZ-91, AZ-61, AZ-92 and AZ-80). These typical magnesium alloys have the following compositions by weight:

Percent ZK-60A:

The alloy AZ-9l is also identified as AZ-91A and comprises a casting alloy; the alloy AZ-6ll corresponds to Dow Alloy K-l and comprises an extrusion alloy; and the alloy AZ-80 corresponds to Dow Alloy O-l and comprises a forging alloy.

PRETREATMENT NO. 1

/2 minute.

(4) Pickle in bath No. 1A at room temperature for a time interval of about 2-3 minutes.

Rinse with water at room temperature for about V2 minute. I

(6)' Pickle in bath No. 2 at room temperature for a time'interval of about 5 minutes.

(7) Rinse with water at room temperature for about /2 minute. 7 (8) Treat in bath No. 3 at a temperature of about 65 C. for a time interval of about 3 minutes. (9) Rinse with water at room temperature for about /2 minute.

PRETREATMENT NO. 2

The work-piece is subjected to the following steps in the order named in this pretreatment procedure:

(1), (2) and (3)4ame as the corresponding steps of Pretreatment No. 1.

(4)Pickle in bath No. 1B at room temperature for a time interval of about 1-1 /z minutes.

(5), (6), (7), (8) and (9)-Same as the corresponding steps of Pretreatment No. 1.

Bath N0. 1A.-This pickling bath employed in Pretreatment No. 1 essentially comprises an aqueous solution of chromic acid, sodium nitrate and nitric acid and 'has the general composition:

1.8 to 2.2 NaNO 0.5 to 0.7 HNO 0.65 to 0.85

The preferred composition of this bath is as follows:

M.p.l. CrO 2 NaNO 0.6 HNO 0.7

This bath may be conveniently formulated by dissolving in water 200 g./l. of CRO and 50 g./l. of NaNO and by adding thereto 50 ml./l. of 42 B (technical) HNO Bath No. 1B.-This pickling bath employed in Pretreatment No.2 essentially comprises an aqueous solution of orthophosphoric acid and has the general composition:

H PO -l2 to 14 m.p.l.

The preferred composition of this bath is as follows:

H PO -13.2 m.p.l.

This bath may be conveniently formulated by adding to one pa-rt of water nine parts of an aqueous solution of 85% H PO Bath N0.2.This pickling bath employed in both Pretreatment No. 1 and Pretreatment No. 2 essentially comprises an aqueous solution of hydrofluoric acid and has the general composition:

HF-6 to 7 /2 m.p.l. The preferred composition of this bath is as follows:

HF6.6 m.p.l. This bath may be conveniently formulated by adding to Water 150 ml./l. of an aqueous solution of 70% HF. Bath N0. 3.--This treatment bath employed in both Pretreatment No. 1 and Pretreatment No. 2 essentially comprises an aqueous solution of an alkali metal pyrophosphate, such as sodium pyrophosphate, and has th general composition:

Na P O O.22 to 0.28 m.p.l.

The preferred composition of this bath is as follows:

NA P O 0.25 m.p.l.

This hath may be conveniently formulated by dissolving in water 65 g./l. of NA P O After the Work-piece has been subjected to Pretreatment No. l or Pretreatment No. 2, depending upon the composition of the magnesium alloy metal surface thereof, it is transferred to a chemical nickel plating bath of the nickel cation-hypophosphite anion type and immersed therein throughout an appropriate time interval in order to effect the plating of a suitable thickness of nickelphosphorus alloy thereupon; which plating bath is normally maintained at a relatively high temperaturein :the general range 94 C. to C., the thickness of the plating being. dependent. upon the time interval of immersion of the work-piece. The plating that is inherently produced by a plating bath of this type essentially comprises about 89% to 97% nickel and about 3% to 11% phosphorus by weight; which plating proceeds at a rate of about 0.9 mil/hour.

This plating bath essentially comprises an aqueous solution of nickel cations, hypophosphite anions, citrate anions and fluoride anions and has the general composition:

l' he preferred composition of this plating bath is as follows:

Ni++ 0.08 m.p.l. (H2POz) m.p.l. Citrate 0.08 m.p.l. F 0.15 m.p.l. Pb++ About 1-p.p.m-. pH 4.6 to 4.7.

This plating bath may be conveniently formulated initially by dissolving in water 0.08 m.p.l. of nickel hypophosphite, about 0.08 m.p.l. of sodium hypophosphite or hypophosphorous acid, about 0.15 m.p.l. of potassium fluoride, and about 0.08 m.p.l. of citric acid, or sodium citrate, together with the introduction of about 1 p.p.m. of. lead ion, and also with the addition of sufficient sodium hydroxide to efiect the pH adjustment into" the range 4.6 to 4.7.

Accordingly, this plating bath as initiallyformulated contained a minimum .amount' of foreign cations (chiefly K+ and' Na*) and no foreign anions; and in passing, it is mentioned that the trace" of lead ions (about I p'.p.m.) serves the stabilizer function, as disclosed in U. S. Patent No. 2,762,723, granted on September 11, 1956,'to Paul Talmey and Gregoire Gutzeit.

In the utilization of this plating bath, nickel cations and hypophosphite anions are depleted, the hypophos-v phite anions being oxidized to phosphite anions; whereby either continuously or periodically, the bath is regenerated in use by the addition of the ingredients noted in order to maintain the ranges thereof set forth. As the phosphite anion builds up in the plating bath, precipitation thereof as nickel phosphite can be prevented by increasing the content of citrate anions.

In carrying out the present process, after the required thickness of the plating has been chemically deposited upon the work-piece in the above described plating bath, it is removed therefrom, rinsed with Water and then sub jected to heat treatment. More particularly, the workpiece and the plating are heated to a temperature of about 400 F. for a time interval of about one hour;

which heat treatment effects intimate bonding between the chemically deposited plating and the metal surface of the work-piece so that a smooth, continuous and uniform coating is provided on the work-piece, that is intimately bonded thereto.

Reconsidering the Pretreatment No. 1 and the Pretreatment No. 2', it is pointed out that magnesium audits alloys, are extremely reactive metals, whichfare subformation of a protective film upon the exterior surface of the magnesium work-piece, which will slow down or prevent this corrosive attack in the plating bath; in other words, the magnesium surface must be passivated, as well ascleaned or pickled.

In the Pretreatment No. "1, the pickling bath No. 1A accomplishes these ends, the protective film being formed fundamentally by the chromic acid and the pickling being effected mostly by the nitric acid, the pickling action of the nitric acid being moderated by the sodium nitrate addition to such extent that when the magnesium oxides are all dissolved from the surface, a stable chromate film is formed thereupon. Thus, in Pretreatment No. 1, pickling bathNo. 1A passivates the pickled magnesium surface by the formation of the stable chromate film thereon.

In the Pretreatment No. 2, the pickling bath No. 1B accomplishes these ends, with the single ingredient, orthophosphoric acid, since this reagent accomplishes both the cleaning or pickling function and the protective filmforming function, the protectivefilm upon the pickled magnesium surface being in the form of a phosphate.

Accordingly, the pickling bath No. 1A and the pickling bath No. 1B possess the common characteristics that they clean the magnesium surface of oxides and deposit a protective film thereupon after the cleaning thereof; whereby the pickled magnesium surface is passivated.

In the process, the subsequent utilization of the pickling bath No. 2 effects an alteration of the film upon the magnesium. surfaces that was deposited thereupon in Pretreatment No. 1 by pickling bath No. 1A or in Pretreatment No. 2 by pickling bath No. 1B; and more particularly, pickling bath No. 2 effects a substitution for the original film mentioned of an insoluble film of magnesium fluoride upon the cleaned metal magnesium surfaces.

Still subsequently in the process, the utilization of the treatment bath No. 3 effects the dissolution of the magriesium fluoride film from the magnesium surface that was deposited thereupon by the pickling bath No. 2, and furtherforrns a final protective phosphate film upon the cleaned magnesium metal surface that is capable of in hibiting attack thereof by the subsequently employed plating bath for a time interval sufficient to bring about the deposit of the nickel-phosphorus alloy upon the magnesium surface from the plating. bath.

.It is emphasized that the pickling bath No. 1A 'of Pre treatment No. 1' is productive of best results' upon the magnesium 'alloys that are low in both aluminum or zinc, while the pickling bath No. 1B of Pretreatment N0. 1 is productive of best results upon the magnesium alloys that are high in either aluminum of zinc.

cReconsidering the plating of the work-pieces after preforeign anions that are normally introduced in such' conventional plating baths by, the utilization of ordinary nickel salts. Specifically, in this plating bath, nickel sulfate, nickel chloride, etc., are not employed as the source ofnickel cations, since the sulfate, chloride, etc., anions are objectionable in substantial quantities, as previously explained.

modified form ,of the pickling bath No. 1 has been successfully employed that essentially comprises an aqueous solution of chromic acid, hydrofluoric acid and nitric acid and has the general composition:

M.p.l.

In this case, the nitric acid is the chief pickling agent, while the hydrofluoric acid serves to inhibit the action of the nitric acid, and the chromic acid is the chief passi vating or film-forming agent; all as previously explained in conjunction with the pickling bath N0. 1A.

Pickling bath No. 1A, as previously described, or the modified form thereof, as set forth immediately above, may be employed alternatively in Pretreatment No. 1; which pickling baths are expressly recommended when the magnesium alloy is low in both aluminum and zinc, as previously explained.

When this particular chemical plating bath is employed, following proper pretreatment as previously explained, the coatings produced upon the work-pieces are smooth, bright, uniform and continuous and intimately bonded thereto and entirely free of pits and blisters. However, the bonding of these improved coatings to the work pieces is substantially improved by the heat treatment, follow-. ing the production of the chemical plating thereupon, at the temperature of about 400 F. for aboutone hour; and in this connection, it is mentioned that this temperature comprises a relatively 'low temperature well below the melting point of the nickel phosphorus alloy that is inherently produced by a chemical plating bath of this character.

, In conjunction with the present process, the plater is warned that satisfactory plating is not produced upon magnesium and its alloys by the utilization of conventional chemical nickel plating baths, even after the utilization of the described pretreatments upon the work-pieces; however, after a definite plating has been obtained upon' a work-piece utilizingthe particular chemical nickel" plat-l ing bath disclosed, the Work-piece may be transferred to a conventional chemical plating bath for the purpose of building-up the plating to a substantial thickness. Specifically, after the work-piece has been plated for a minimum time interval of about twenty minutes, it may be transferred to a conventional chemical nickel plating bath for the purpose of building-up the thickness of the coating, without the sacrifice of adhesion in the ultimately produced coating. In further explanation of these points, it is mentioned that unless the particular chemical plating bath described is employed for a minimum time interval of about twenty minutes prior to the transfer of the work-pieces to a conventional chemical plating bath, the coatings are blistered and the work-pieces are badly etched at any cracks or discontinuities in the coatings. Moreover, the heat-treatment described should be employed, since it materially increases the adhesion of the coatings upon the work-pieces.

The following test data demonstrates the critical character of the pretreatment in the present process; and in the production of these data, all of the work-pieces are given the required pretreatment, as previously explained, but only as a routine matter and deliberately such that the pretreatmentwas not necessarily entirely thorough and complete. This procedure was employed for the very purpose of determining the reliability of the present, process, when it was employed on a production line basis, whereinit was inherent that all of the work-pieces were not necessarily thoroughly and completely cleaned and activated. The test data are as follows:

In these test data, all of the blistered coatings on the work-pieces were directly traceable to lack of proper pretreatment of the work-pieces; and perfect platings were always obtained when the pretreatment was entirely thorough and complete as established by numerous tests, wherein each of the various work-pieces was carefully pretreated and thoroughly inspected following the pretreatment.

It is reiterated that the data above set forth in the table is for the purpose of demonstrating the practical application of the present process, since it is apparent that employing the production line technique, at least 90% of the work-pieces receive proper pretreatment.

In passing, it is noted that a defective coating may be readily stripped electrolytically from a work-piece of the character described employing an aqueous bath containing about 150 g./l. of potassium fluoride and utilizing a potential of about 6 volts. Preferably, the work-piece is suspended in the electrolytic solution utilizing a magnesium hook that is connected as the anode in the electric circuit. Of course, after electrolytic stripping, the work-piece should be subjected to proper pretreatment, preceding another attempt of chemical nickel plating thereof in accordance with the present process.

In view of the foregoing, it is apparent that there has been provided an improved process of producing coatings of nickel-phosphorus alloy upon work-pieces having surfaces formed of magnesium and its alloys, which coatings are smooth, bright, uniform, continuous and intimately bonded to the work-pieces. Moreover, the method involves improved pretreatment steps, plating steps and heat treatment steps that are correlated and coordinated to obtain the results aforesaid.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is' claimed is:

. l. The process of plating. with nickel the metal surface of a work-piece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution of a passivating agent selected from the class consisting of chromic acid and orthophosphoric acid in order to form a protective film thereupon to prevent corrosive attack thereof by water, then treating said metal surface with a second aqueous solution of a fluoride in order to form a fluoride film thereupon, then treating said metal surface with a third aqueous solution of a pyrophosphate in order to forma phosphate film thereupon, and then contacting said metal surface with an aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type also containing a citrate and a fluoride.

' 2. The process set forth in claim 1, wherein said second aqueous solution essentially comprises hydrofluoric acid.

B'J'I'he process set forth in claim 1, wherein said aqueous solution essentially comprises alkali metal pyrm phosphate.

4. The process set forth in claim 1, and further com prising thereafter heat-treating said work-piece at a temperature of about 400 F. for a time interval of about one hour.

5. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface 'is formed essentially of magnesium; said process comprising clean ing said metal surface, then treating said metal surface with a first aqueous solution of chromic acid in order to form a chromate film thereupon, then treating said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, then treating said metal surface with a third aqueous solution of a pyrophosphate in order to form a phosphate film thereupon, and then contacting said treated metal surface with an aqueous chemical nickel plating bath of the nickel cation-hypophosphite anion type also containing a citrate and a fluoride.

6. The process set forth in claim 5, and further comprising thereafter heat-treating said work-piece at a temperature of about 400 F. for a time interval of about 1 hour.

7. The process set forth in claim 5, wherein the metal surface of said work-piece comprises an alloy of about 3% aluminum, 1% zinc, and 96% magnesium.

8. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, rinsing said metal surface, then treating said metal surface with a first aqueous solution comprising about 2 m.p.l. of chromium trioxide and about 0.6 m.p.l. of alkali metal nitrate and about 0.75 m.p.l. of nitric acid in order to form a chromate film thereupon, rinsing said metal surface, then treating said metal surface with a second aqueous solution of about 6.6 m.p.l. of hydrofluoric acid in order to form a fluoride film thereupon, rinsing said metal surface, then treating said metal surface with a third aqueous solution of about 0.25 m.p.l. of alkali metal pyrophosphate in order to forma phosphate film thereupon, rinsing said metal surface, and then immersing said metal surface in an aqueous chemical nickel plating bath, said bath comprising about 0.08 m.p.l. of nickel cations, about 0.24 m.p.l. of hypophosphite anions, about 0.08 m.p.l. of citrate anions, and about 0.15 m.p.l. of fluoride anions, said bath having a pH in the range 4.6 to 4.7.

9. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, rinsing said metal surface, then treating said metal surface with a first aqueous solution comprising about 2.4 m.p.l. of chromium trioxide and about 0.07 m.p.l. of hydrofluoric acid andabout 0.15 m.p.l. of nitric acid in order to form a chromate film thereupon, rinsing said metal surface, then treating. said metal surface with a second aqueous solution of about 6.6 m.p.l. of hydrofluoric acid in order to form a fluoride film thereupon, rinsing said metal surface, then treating said metal surface with a third aqueous solution of about 0.25 m.p.l. of alkali metal pyrophosphate in order to form a phosphate film thereupon, rinsing said metal surface, and then immersing said metal surface in an aqueous chemical nickel plating bath, said bath comprising about 0.08 m.p.l. of nickel cations, about 0.24 m.p.l. of hypo phosphite anions, about 0.08 m.p.l. of citrate anions, and about 0.15 m.p.l. of fluoride anions, said bath having a pH in the range 4.6 to 4.7.

10. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface is formed essentially of magnesium; said process comprising clean ing said metal surface, then treating said metal surface with a first aqueous solution of orthophosphoric acid in order to form a phosphate film thereupon, then treating 9. said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, then treating said metal surface with a third aqueous solutionof 'apyrophosphate in order to form a phosphate film thereupon, andthen contacting said treated metal surface with an aqueous chemical nickel plating bath ofthe nickel cation-hypophosphite anion type also containing a citrate and a fluoride. p

11. The process set forth in claim' 10, and further comprising thereafter heat-treating said work-piece at a temfierature of about 400 F. for a time interval of about 1 our.

12. The process set forth in claim 10, wherein the metal surface of said work-piece comprises an alloy of about 5 /5 zinc, V2 zirconium, and the balance mainly magnesium.

13." Ihe process set forth in claim 10, wherein the metal surface of said work-piece comprises an alloy of about 6 to aluminum, /2 to 1% zinc, and the balance mainly magnesium.

.14. The process of plating with nickel the metal surface of a work-piece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, rinsing said metal surface, then treating said metal surface with a first aqueous solution comprising about 13.2 m.p.l.-of orthophosphoric acid in order to form a phosphate film thereupon, rinsing said metal surface, then treating said metal surface with a second aqueous solution of about 6.6 m.p.l. of hydrofluoric acid in order to form a fluoride film thereupon, rinsing said metal surface, then treating said metal surface with a third aqueous solution of about 0.25 m.p.l.

, of alkali metal pyrophosphate in order to form a phosphate film thereupon, rinsing said metal surface, and then immersing said metal surface in an aqueous chemical nickel plating bath, said bath comprising about 0.08 m.p.l. of nickel cations, about 0.24 m.p.l. of hypophosphite anions, about 0.08 m.p.l. of citrate anions, and about 0.15 m.p.l. of fluoride anions, said bath having a pH in the range 4.6 to 4.7.

15. The process of preparing the metal surface of a work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution of chromium trioxide in order to form a chromatic film thereupon, then treating said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of a pyrophosphate in order to form a phosphate film thereupon.

16. The process set forth in claim 15, wherein said second aqueous solution comprises hydrofluoric acid in the general range 6 to 7 /2 moles per liter.

17. The process set forth in claim 15, wherein said second aqueous solution comprises about 6.6 m.p.l. of hydrofluoric acid.

18. The process set forth in claim 15, wherein said third aqueous solution comprises alkali metal pyrophos phate in the general range 0.22 to 0.28 mole per liter.

19. The process set forth in claim 15, wherein said third aqueous solution comprises about 0.25 m.p.l. of alkali metal pyrophosphate.

20. The process of preparing the metal surface of a work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution of chromium trioxide and an alkali metal nitrate and nitric acid in order to form a chromate film thereupon, then treating said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of an alkali metal pyrophosphate in order to form a phosphate film thereupon;

, 21. The process set forth in claim 20, wherein said first aqueous solution comprises chromium trioxide in the general range 1.8 to 2.2 moles per liter, alkali metal nitrate in the general range 0:5 to 0.7 mole per liter, and nitric acid in the general range 0.65 to 0.85 mole per liter. r

:22. The process set forth-in claim 20, wherein said first aqueous solution comprises about 2 m.p.l. of chromium t-rioxide, about 0.6 m.p.lrof metal nitrate, and about 0.75 m.p.l. of nitric acid.

23. The process of preparing the metal surface of a work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of mag nesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution comprising about 2 m.p.l. of chromium trioxide and about 0.6 m.p.l. of alkali metal nitrate and about 0.75 m.p.l. of nitric acid in order to form a chromate film thereupon, then treating said metal surface with a secondaqueous solutionof about 6.6 m.p.l. of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of about 0.25 m.p.l. of alkali metal pyrophosphate in order to form .a phosphate film thereupon.

'24. The process of' preparing the metal surface ofa work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution of chromium trioxide and hydrofluoric acid and nitric acid in order to form a chromate film thereupon, then treating said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of an alkali metal phyrophosphate in order to form a phosphate film thereupon.

25. The process set forth in claim 24, wherein said first aqueous solution comprises chromium trioxide in the general range 2.0 to 2.8 moles per liter, hydrofluoric acid in the general range 0.10 to 0.20 mole per liter, and nitric acid in the general range 0.05 to 0.10 mole per liter.

26. The process set forth in claim 24, wherein said first aqueous solution comprises about 2.4 m.p.l. of chromium trioxide, about 0.07 m.p.l. of hydrofluoric acid, and about 0.15 m.p.l. of nitric acid.

27. The process of preparing the metal surface of a work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution of orthophosphoric acid in order to form a phosphate film thereupon, then treating said metal surface with a second aqueous solution of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of a pyrophosphate in order to form a phosphate film thereupon.

28. The process set forth in claim 27, wherein said first aqueous solution comprises orthophosphoric acid in the general range 12 to 14 moles per liter.

29. The process set forth in claim 27, wherein said first aqueous solution comprises about 13.2 m.p.l. of orthophosphoric acid.

30. The proces set forth in claim 27, wherein said second aqueous solution comprises hydrofluoric acid in the general range 6 to 7% moles per liter.

31. The process set forth in claim 27, wherein said in a a... 1 .11 second; aqueous solution comprises about 6.6 m.p.l. of hydrofluoric acid.- Y W V 32.- The process set forth in claim 27, wherein said third aqueous solution comprises alkali metal pyrophosphate in the general range 0.22 to 0.28 mole per liter.

33. The process set forthin claim 27, whereinsaid thirdaqueous solution comprises about 0.25 m.p.l. of alkali metal pyrophosphate. V

34 The process of preparing the metalsurface of a work-piece for chemical nickel plating with an aqueous bath of the nickel cation-hypophosphite anion type, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning said metal surface, then treating said metal surface with a first aqueous solution comprising about 13.2 m.p.l. of orthophosphoric acid in order to form a phosphate film thereupon, then treating said metal surface with a second aqueous solution of about 6.6 m.p.l. of hydrofluoric acid in order to form a fluoride film thereupon, and then treating said metal surface with a third aqueous solution of about 0.25 m.p.l. of alkali metal pyrophosphate in order to form a phosphatefilm thereupon.

35. The process of increasing the thickness of a nickel coating carried upon the surface of a workpiece formed essentially of magnesium, said process comprising contacting during a time interval said nickel coating with an aqueouschemical nickel plating bath, said bath consisting essentially of about 0.08 m.p.l. of nickel hypophosphite, about 0.16 m.p.l. of hypophosphorus acid, about metal fluoride, said bath having a pH in the range 4.6- to 4.7. v

References Cited in the file of this patent UNITED STATES PATENTS 1,014,454 Clark Jail. 9 1912 1,818,008 Rit'ter et a1. Aug. '11, 1931 2,314,565 Thompson Mar. 23, 1943 2,428,749 De Long O t. 7, 1947 2,458,073 Henrker et al. Jan. 4, 1949 2,588,234 Henricks Mar. 4, 1952 2,694,019 Gutzeit NOV. 9, 19 54 2,738,289 Hodge Mar. 13, 1956 2,762,723 Talmey et al.- Sept. 11, 1956 2,766,138 Talmey Oct. 1956: 2,774,688 Girard D66. 18, 1956 2, 00,422 Piccinelli- July' 23, 1957 2,819,187 Gutzeit et a1. Jan. 7, 1958 2,833,667 Dalton May 6, 1958 2,856,322 Parson et al. Oct. 14, 1958 2,886,451 Budininkas May 12,1959

FOREIGN PATENTS 607,404 Great Britain Aug. 31, 1948' OTHER REFERENCES Research paper RP 1835, vol. 39, November 1947, Journal of Research of National Bureau of Standards,

0.08 m.p.l. of citric acid, and about 0.15 m.p.l. of alkali p g 38 to 39 p 3 particularly relied UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 2 983 634 May 9 1961 Pranas 'Budininkas It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent. should read as "corrected below Column 9 lines 47 and 48 for "-chro=mal;ic' read chromalze column 1O line 72 for "proces" read process Signed and sealed this 24th day of October 19610 (SEAL) Attest:

ERNEST W. SWIDER Commissioner of Patents USCOMM-DC- 

1. THE PROCESS OF PLATING WITH NICKEL THE METAL SURFACE OF A WORK-PIECE, WHEREIN SAID METAL SURFACE IS FORMED ESSENTIALLY OF MAGNESIUM, SAID PROCESS COMPRISING CLEANING SAID METAL SURFACE, THEN TREATING SAID METAL SURFACE WITH A FIRST AQUEOUS SOLUTION OF A PASSIVATING AGENT SELECTED FROM THE CLASS CONSISTING OF CHROMIC ACID AND ORTHOPHOSPHORIC ACID IN ORDER TO FORM A PROTECTIVE FILM THEREUPON TO PREVENT CORROSIVE ATTACK THEREOF BY WATER, THEN TREATING SAID METAL SURFACE WITH A SECOND AQUEOUS SOLUTION OF A FLUORIDE IN ORDER TO FORM A FLUORIDE FILM THEREUPON, THEN TREATING SAID METAL SURFACE WITH A THIRD AQUEOUS SOLUTION OF A PYROPHOSPHATE IN ORDER TO FORM A PHOSPHATE FILM THEREUPON, AND THEN CONTACTING SAID METAL SURFACE WITH AN AQUEOUS CHEMICAL NICKEL PLATING BATH OF THE NICKEL CATION-HYPOPHOSPHITE ANION TYPE ALSO CONTAINING A CITRATE AND A FLUORIDE. 